Stem cells are found as units of organization, generation, and regeneration in virtually all metazoan species. Both germline and somatic tissues are organized in a stem/progenitor/progeny plan of cell lineage development. Hematopoietic stem cells (HSC) are the only functional cells in human bone marrow/mobilized peripheral blood transplantation, and it is likely that other organ and tissue-specific cell therapies will depend on their content of stem cells for their regenerative capacities. We have recently found that in a protochordate species, Botryllus schlosseri, that both germline and somatic cell progenitors-probably stem cells-can migrate between genetically distinct individuals sharing a natural vascular bridge and replace host cells in the generation (or regeneration) of gonads and somatic organs respectively. The predilection to win in these competitions is genetically determined, and therefore stem cells are likely units of natural selection also. Here we embark on a series of experiments to create the assays and the tools (mainly monoclonal antibodies to separate putative stem cells and to characterize their progeny) to isolate each of these Botryllus stem cells. In parallel we plan to define the forms of Mendelian inheritance that characterize somatic and germline cell competitive proclivities. These experiments will serve as a prelude to develop an understanding of the phylogeny of stem cells, the genetic bases for their functions, and the eventual identification of the human homologues of these Botryllus genes.